SERVER-58579 refactored the BucketUnpacker to use an `UnpackerImpl` interface to support a new `BSONColumn` type to support compression of time series columns. When SERVER-58579 landed it wasn't clear how we can best share the bucketUnpacker in both branches of the TrialStage,

https://github.com/10gen/mongo/pull/426#discussion_r697471248

While the work done in each TrialStage might be a little different, if they share the same `BSONColumn` then any bucket that has been previously unpacked and hence uncompressed will be cached and should give a perf gain. We should see how we can best share the `BSONColumn` across multiple BucketUnapckers.